Corrosion is a process that involves two simultaneous reactions which are called half-cells. One half-cell reaction is the oxidation or corrosion of the metal. This process involves the loss of electrons, e.g., EQU 2M.fwdarw.2M.sup.+ +2e.sup.-
Electrons from the oxidation process are in turn used by an associated reduction half-cell reaction, which is often the reduction of oxygen or hydrogen, e.g., EQU O.sub.2 +4H.sup.+ +4e.sup.-.fwdarw.2H.sub.2 O EQU 2H.sup.+ +2e.sup.-.fwdarw.H.sub.2
The oxidation reaction (corrosion process) can only proceed at a rate governed by the reduction reaction that uses the electrons from the oxidation process. This is because charge neutrality must be maintained.
Galvanic corrosion occurs when two dissimilar metals make contact with one another in the presence of an electrolyte thereby forming a galvanic couple. The more noble metal (more cathodic on the galvanic series) provides additional surface area for the reduction reaction to occur on. This accelerates the oxidation/corrosion of the less noble metal (more anodic on the galvanic series). The extent of corrosion is greatest at the interface of the two metals, but may also occur at some distance away from the actual interface.
Heat exchangers made of copper tubes and aluminum fins have about ten times lower corrosion durability as compared to those made with aluminum fins and aluminum tubes. This, as discussed above, is because when two dissimilar metals (like copper and aluminum) are in contact with each other in a corrosive environment, a galvanic couple forms and the more noble metal promotes the corrosion of the more active metal. The adverse role of the more noble metal, copper in this application, is that: (1) it provides additional surface area for the reduction reaction of the corrosion process to occur on, and (2) corrosion reduction reaction rates are very high on copper. Both of these factors accelerate the corrosion process and result in a ten-fold increase in corrosion rate of the aluminum fins.
In coastal regions, the most common electrolyte is salt water. A fine salt water mist may be blown inland for up to fifty miles from the coast. Sulfur dioxide and other industrial pollutants also creates an electrolyte when combined with moisture.
A common method of preventing galvanic corrosion has been to coat the exposed surfaces of the corroding metal with various types of paint. These protective coatings have met with only limited success for a number of reasons. The main problem with these types of coatings is that their effectiveness at preventing corrosion is degraded by exposure to the environmental elements such as ultraviolet light and acid rain. Another common problem is that the coating materials often do not adhere well to the metal substrates and eventually flake off or erode away exposing the metal substrates. Moreover, such protective coatings are somewhat porous and allow the electrolyte to penetrate the surface of the substrates and connect the galvanic couple. Such coatings can also be relatively expensive.
When using aluminum fins and copper tubes, coating the aluminum fin stock is a current prevalent option. However, this approach decreases the thermal efficiency of a heat exchanger because it inserts a thermal conduction barrier between the fin and the tube. It also has the potential problems of: (1) rapid corrosive attack at the bare aluminum sheared edges near the fin collar due to the large increase in the ratio between the copper and aluminum surfaces, (2) increased tool wear and fouling during the fin stamping process, and (3) added costs.
The current art of protective coatings is to protect a metal that corrodes by covering it with a coating or paint that shields it from the aggressive environment. This concept is well known and involves coatings such as paints or metallic coatings that protect the substrate such as galvanized steel. In the case of galvanized steel the zinc coating is also sacrificial to the steel substrate. For galvanic couples, the two metal combinations can be made more compatible by selecting metals near or similar to each other on the galvanic series, which effectively reduces the driving force for galvanic corrosion.